1. Field of the Invention
This invention relates to a telephone instrument, and more particularly to a cordless telephone instrument.
2. Description of the Related Arts
A cordless telephone instrument is constituted from a fixed unit and a portable unit connected to the fixed unit by way of a radio channel. With the cordless telephone instrument, service can be performed between the fixed unit and the portable unit while the portable unit is being moved within an area within which the fixed unit and the portable unit can be connected to allow operation between them through the radio channel, that is, within an operational area. Accordingly, when the portable unit is moved outside the operational area, connection between the fixed unit and the portable unit through the radio channel is disabled and a call through a telephone line by the portable unit becomes disabled. Therefore, in some conventional cordless telephone instruments, an alarm is developed when it is detected during talking that the portable unit has moved out of the operational area, as disclosed in Japanese Patent Application Laid-Open No. Heisei 3-114338 or 3-114339. In some other conventional cordless telephone instruments, if no response is received from a fixed unit when the portable unit tries to originate a call, it is determined that the portable unit is outside the operational area, and an alarm is developed. Further, in some other conventional cordless telephone instruments, if no response is received from a portable unit within a predetermined period of time another more propagative call signal is developed from the fixed unit, as disclosed in Japanese Patent Application Laid-Open No. Heisei 3-13040.
In this manner, various cordless telephone instruments wherein it is displayed by way of an alarm whether the portable unit is positioned within the operational area have conventionally been proposed. However, with the cordless telephone instruments of the type described, since determination whether or not the portable unit is positioned within the operational area is performed during talking or call originating operation or call receiving operation, it is not able to notify to the user whether the telephone service is available when the cordless telephone instrument is in a state of on hook which occupies the major part in normal state. Accordingly, it sometimes occurs that, while the cordless telephone instrument is not in use, the portable unit is carried to a location outside the operational area and cannot originate a call at the location, or since the portable unit is outside the operational area upon reception of a call, the reception of a call is not known to the user, resulting in a miss of the timing to respond. In this manner, when the portable unit should be used, sometimes it actually cannot be used. Consequently, the conventional cordless telephone instruments described above are disadvantageous to enjoy sufficiently the merit of a cordless telephone instrument such that the portable unit can be freely carried and used within a certain area.